Normally it will say "anti-spin" after the gear ratio, so it is probably not a limited slip.

 

I posted a thread about this a few months ago after using my owners manual as reading material. The thread I posted didn't leave a real answer and I am still curious about this. It's odd to me as trucks mid 90s and earlier and even modern 3/4 and 1 ton trucks all use 75-90 and seem to have no issues with the diff. Meanwhile my truck, which uses 75-140 is in the shop having all the carrier, pinion and axle bearing replaced, $1000 or so for that even after using the horribly over priced "spec". That was my whining noise for those who read my "whine" thread. All bearing except the inside pinion was either smoked or close to it.

 

So you used 75w-90 or 75w-140?

Because I was going to just use what book says which is 75w-90 but I believe someone in this thread mentioned using the 75w-140 in the rear

 

Also would it be ok to do 75w-90 in front as well even though it calls for 80w-90?

 

I was using 75-140. I beleive it's going to 75-90 from here on out. Dodge has used the 9.25 rear diff in trucks and cars for many years, and I haven't heard of that much trouble from them except the 3rd gen Rams. Its my understanding that nothing has changed in the diff between 02 and 03 but a different fluid is specified for the 03 and back spec'd for the 02 and all of a sudden it's common place for burned up pinion bearings. I don't know if the 75-140 I too thick but aside from a wrong diff setup and bearing per load, they really shouldn't go out given that they are bathed in oil all the time, and especially not in a ram application where dodge wants you to change the fluid every 30k miles.

 

So I guess my remaining question is would 75w-90 be ok in the front also...?

 

OK so here goes....
They changed spec because of the HP and TQ increase for starters. More power means more heat and the need for more lubrication. SO, with heat, the fluid becomes less viscous, this is why the upgrade from 75w-90 to 75w-140. The 90 is more "watery" at operating temps than the 140 is, so the 90 gets slung off the gears much easier not allowing the fluid to do its job. This does 2 things...1)doesn't allow proper lubrication and 2)allows the gears to overheat. Overheating means expansion and expansion means wear. This is another reason why the spec calls for Full Synthetic. Full Synthetic has smaller molecules and can squeeze into smaller spaces allowing better lubrication and penetration.
Synthetic oils increase hyoind and worm gear efficiency by 30%, reduce the overall heat because the polymers reduce the coefficient drag due to anit-scuffing agents. Synthetic oils also maintain their VI rating much better than standard oils do. So, when the differential temperatures fluctuate, you get far less fluctuation in the visccousness of the fluid itself. This makes the fluid much more stable at all temperatures. It maintains consistency and allows the fluid to last MUCH longer.
Chrysler also changed the fluid specs. Going from 9/16" below fill hole to 1/4" below fill hole.
I think the difference is the design of the differential internal parts and use of bearings that are manufactured!
The reason the front diff calls for a lighter fluid than the rear is because it's under far less stress and the intervals between actual use is low. A lighter fluid upfront reduces drag while in 2wd. Spin a hot gearset by hand with 75w-90 and then again with 75w-140 and you'd be amazed on the difference.
Do I think you could get away with 75w-90 Synthetic in the front, yes because the Synthetics are much more stable at all operating temps than any mineral oil would be, but I certainly wouldnt put in out back! 80w-90 has twice the VI rating at cold temps than 75w-90 does! 75w-140 has twice the VI rating at operating temps as 75w-90 does!
If your not towing, you can get away with lower VI indexed oils. If you tow or haul anything, I would put 75w-140 out back regardless of what the manual states!!!

HERE'S a chart to show you VI comparison between the oil weights.

 

I appreciate you explaining this and stating a valid point as to why. I was beginning to think that the 75-140 may have been to thick and not able to lubricate properly. I'm a fan of synthetic oils no matter which way I go but in this case I was beginning to become concerned with the viscosity.

 

Glad I ordered fluid based off the 03 specification. Good info.

 

I don't tow very often maybe 3 times a year max and during winter I pull people out that get stuck but that's not often either

So is Redline a good brand? I was going to order 5 quarts of the 75w-90 Heavy Shock Proof oil